Thermally responsive bimetallic members that exhibit a snap-action response are commonly utilized to actuate overheat protection and thermostatic switching mechanisms. One type of such mechanisms is a thermostatic switch that utilizes an actuator formed of a bimetallic material having materials of relatively low and high thermal expansion coefficients joined together along a common interface. Snap-action bimetallic switching mechanisms typically exhibit two states of stability with each of these states being responsive to a predetermined threshold or set-point temperature. When the switching mechanism senses a temperature that is below a first lower of these predetermined set-point temperatures, the thermally responsive member is in one of the two stable states. Accordingly, when the sensed temperature is above a second higher predetermined set-point temperature, the thermally responsive member snaps to a second of the two stable states and remains in this second state while the sensed temperature remains at or above this second higher set-point temperature. Should the sensed temperature be reduced to the first lower temperature, the temperature of the member is lowered correspondingly. As a result, the thermally responsive member snaps back to the first lower temperature state. The difference between the two predetermined set-point temperatures corresponding to the respective first and second states of stability is known as the “differential temperature” of the thermally responsive member.
A known method of manufacturing thermally responsive snap-action switches of the variety described above has included a forming operation in which a pre-sized blank of the thermally responsive bimetallic member is positioned between two opposingly positioned shaping or die members. The shaping members are actuated to engage the bimetallic member, thereby providing the bimetallic member with the desired configuration needed to achieve snap-action at each of the two predetermined set-point temperatures. Such a configuration usually consists of a knee and/or corresponding bowed portion, a dimpled portion or portions, or a series of ridges. Examples of such of formations are described in U.S. Pat. No. 3,748,888 and U.S. Pat. No. 3,933,022, each of which is incorporated herein by reference in its entirety, wherein a thermally responsive snap-action bimetallic disc is provided.
U.S. Pat. No. 3,748,888 also describes a smoothly formed prior art disc-shaped snap-action bimetallic member, as illustrated in side view in FIG. 1. A bimetallic member 1 is formed using a disc of material formed of two materials 2, 3 having different thermal expansion coefficients joined together along contiguous surfaces. One of the members 2 is formed of a material having a relatively high coefficient or rate of thermal expansion, while the other member 3 is formed of a material having a low coefficient or rate of thermal expansion relative to that of the first member 2. The difference in thermal expansion coefficients between the two members 2, 3 is a factor in determining the set-point temperature at which the resulting bimetallic disc actuator 1 operates and in an actuation force F produced by the snap-action. The disc-shaped bimetallic member 1 is often circular and, in some instances, is provided with a small, centrally located aperture therethrough (not shown). Bimetallic discs of this type are generally formed by “bumping” a flat circular disc blank with a punch-and-die set to stretch the bimetallic material of the disc into the concave structure having a depth H1, as illustrated by full line 4 in FIG. 1. The bimetallic disc 1 is formed, for example, with a substantially planar peripheral hoop portion 5 surrounding a central portion 6 stretched into a concave configuration. The central portion 6 is mobile relative to the peripheral hoop portion 5, the central portion 6 moving from one side of the peripheral hoop portion 5 to the other as a function of temperature. The set-point operation temperature and the force F applied by the snap-action are thus physical characteristics of the two members 2, 3 that form the bimetallic member 1.
Generally, when the bimetallic disc 1 is intended to operate at a temperature above ambient temperature, the disc 1 is bumped on the high expansion rate side 2 to form the central stretched portion 6, whereby the central portion 6 is stretched to space the inner concave surface thereof to a depth H1 away from the plane P of the peripheral hoop portion 5, as illustrated by the full line configuration 4. The depth of penetration of the punch during the bumping operation determines the depth H1 and thus is another factor in determining both the upper set-point temperature and the force F applied by the snap-action operation of the disc 1. The set-point operation temperature and the force F applied by the snap-action are thus also structural characteristics of the bimetallic member 1, as also described in above-incorporated U.S. Pat. No. 3,748,888.
The bimetallic disc 1 is illustrated in FIG. 1 in full line 4 in one of its two states of stability. Assuming the bimetallic disc 1 is intended for operation at a set-point temperature above ambient temperature, the high expansion rate side is located on the surface 2 and the low expansion rate side is along the surface 3. If the bimetallic disc 1 is intended for operation at a set-point temperature below ambient temperature, the bimetallic disc 1 is formed in the opposite shape with the low expansion side located on the surface 2 and the high expansion rate side along the surface 3. For purposes of explanation only, the bimetallic disc 1 shown in FIG. 1 is assumed to be intended for operation at a set-point temperature above ambient temperature. Accordingly, at a temperature well below the upper set-point temperature the bimetallic disc 1 is configured with the central stretched portion 6 in an upwardly concave state, as shown by the upper dotted line 7.
As the temperature of the bimetallic disc 1 is raised to approach its upper set-point operating temperature, the high expansion rate material 2 begins to stretch, while the lower expansion rate material 3 remains relatively stable. As the high expansion rate material 2 expands or grows, it is restrained by the relatively more slowly changing lower expansion rate material 3. Both the higher and lower expansion rate sides 2, 3 of the bimetallic disc 1 become distorted by the thermally induced stresses, and the central mobile portion 6 of the bimetallic disc 1 changes configuration with a slow movement or “creep” action from the upper dotted line configuration 7 to the full line configuration 4. The inner concave surface of the central mobile portion 6 is spaced the depth H1 away from the plane P of the peripheral hoop portion 5. The full line configuration 4 is considered herein to be a first state of stability.
As soon as the temperature of the bimetallic disc 1 reaches its upper predetermined set-point temperature of operation, the central stretched portion 6 of the disc 1 moves with snap-action downward through the unstretched hoop portion 5 to the second state of stability with the inner concave surface of the central mobile portion 6 spaced a distance H2 away from the plane P of the peripheral hoop portion 5, as shown by the phantom line 8. If the temperature of the bimetallic disc 1 is raised to a still higher temperature, the high expansion rate material 2 continues to expand at a greater rate than the relatively lower expansion rate material 3 joined thereto. As a result of this continued differential expansion, the central mobile portion 6 of the bimetallic disc 1 continues to creep toward a state of even greater downward concavity, as shown by the second lower dotted line configuration 9.
As the temperature of the bimetallic disc member 1 is reduced form the high temperature toward the lower predetermined set-point temperature of operation, the central mobile portion 6 of the bimetallic disc 1 moves from the state of extreme concavity, as shown by the lower dotted line 9, toward the second state of stability indicated in phantom 8. As the temperature of the bimetallic disc 1 is reduced below the second or lower predetermined set-point temperature of operation, the material 2 having the relatively larger thermal coefficient also contracts or shrinks more rapidly than the other material 3 having the relatively smaller thermal coefficient. The bimetallic disc 1 changes configuration with a similar slow movement or creep action from the state of greatest downward concavity toward the second state of stability indicated in phantom 8. As the bimetallic disc 1 reaches the lower set-point temperature, the central stretched portion 6 snaps back through the unstretched hoop portion to the first state of stability, as shown by the upper full line 4. If the temperature is decreased still further, the differential expansion between the high and low rate materials 2, 3 causes the central mobile portion 6 to continue to creep toward the state of greatest upward concavity, as shown by the upper dotted line 7.
Many thermal switch designs use one of the bimetallic discs 1 that snap into a different state of concavity at a predetermined threshold or set-point temperature, thereby closing a contact or other indicator to signal that the set-point has been reached. The speed at which the bimetallic disc actuator 1 changes state is commonly known as the “snap rate.” As the term implies, the change from one bi-stable state to the other is not normally instantaneous, but is measurable. A slow snap rate means that the state change occurs at a low rate of speed, while a fast snap rate means that the state change occurs at a high rate of speed. Accordingly, in some known configurations of switch and indicator devices, a slow snap rate results in arcing between the operative electrical contacts. Slow snap rates thus limit the current carrying capacity of the thermal switch or indicator device. In contrast, a fast snap rate means that the change in state occurs rapidly, which increases the amount of current the thermal switch or indicator device can carry without arcing. The temperature rate of change affects the snap rate. A slower temperature rate of change tends to slow the snap rate, while a faster temperature rate of change usually results in a faster snap rate. While some applications provide fast temperature rates, switches and indicators experience very slow temperature rates in many other applications. In some applications, the temperature rates may be as low as about 1 degree F. per minute or less. For long-term reliability the device must operate in these very slow temperature application rates without arcing.
Furthermore, a minimum force F is required to actuate the contacts. As described above, the force F is thermally induced in the bimetallic disc 1 as the result of both the depth H1 of the concavity formed in the disc 1, and the differential thermal expansion rate between the high and low expansion rate sides 2, 3 thereof The force F produced during transit from one state of stability to the other state must be sufficiently powerful to overcome the contact restoring force in order to actuate the device. For example, the force F must be sufficient to overcome a restoring spring force in a flexible switch contact. If a bimetallic disc 1 with insufficient snap force F is installed into a thermal switch or other indicator device, the switch or device may fail prematurely, requiring replacement of the bimetal disc 1 or replacement of the entire mechanism.
Typically, the snap force F generated by the individual bimetallic disc 1 is tested prior to installation in the using device. For example, the bimetallic discs 1 are pre-tested under maximum load to ensure that each exerts sufficient snap force F at temperature application rates of about 1 degree F. per minute or less to actuate the device's contact without arcing. One known method of ensuring the snap quality of the bimetallic disc 1 is testing of the force F produced during actuation of the snap in situ. Pre-testing is thus accomplished by placing the disc 1 in the intended device and testing the fully assembled thermal switch or other indicator mechanism. Pre-testing is thus cumbersome and time consuming. Furthermore, the present in situ testing process is typically a simple go/no-go test in which marginally performing bimetallic discs 1 may remain undiscovered. The manufacturer may thus be forced to employ excessively conservative quality control measures.